Magnetic (or magnetoresistive) random access memory (MRAM) is a non-volatile solid-state memory employing magnetoresistive (MR) elements or magnetic tunnel junctions (MTJs) to store and retrieve data. The MR element uses a tunnel (or giant) magnetoresistive effect in magnetic multilayers. The MR element includes at least two magnetic layers separated from each other by a thin nonmagnetic insulator or semiconductor layer that serves as a tunnel barrier layer. One of the magnetic layers having a fixed magnetization direction is called a pinned (or reference) layer. Another magnetic layer having a reversible magnetization direction is called a free (or storage) layer. Resistance of the MR element depends on a mutual direction of the magnetizations in the free and pinned layers. The resistance is low when the magnetization directions are parallel to each other and high when they are anti-parallel. Parallel configuration of the magnetization directions corresponds to a logic “0”. The anti-parallel configuration of the magnetizations corresponds to logic “1”. A difference in the resistance between two logic states can exceed several hundred percents at room temperature. Theory predicts that MR ratio in MTJs made of FeCo/MgO/FeCo multilayer having body-centered cubic (bcc) structure with preferred (001) orientation can exceed 1000% at room temperature. The high TMR is a result of coherent tunneling of highly spin-polarized conductance electrons through the multilayer structure.
The magnetization direction in the free layer can be reversed by an external magnetic field or by a spin-polarized current running through the MR element in a direction perpendicular to its surface (substrate). Reversal of the magnetization direction in the MR element by a spin-polarized current (spin momentum transfer or spin torque transfer) are widely used in MRAM technology. The magnetization direction in the free layer can be controlled by a direction of the spin-polarized current running through the MR element, for instance, from the pinned layer to the free layer or vice-versa.
CoFeB/MgO/CoFeB multilayer has became a system of choice for manufacturing of MTJs. The multilayer having amorphous structure in as-deposited state can be crystallized into coherent body-centered cubic (bcc) structure with a (001) plane oriented. Moreover MgO can form flat and sharp interface with CoFeB layer. That is essential for coherent tunneling of spin-polarized electrons providing high tunneling magnetoresistance (TMR) and low density of switching current.
CoFeB layers typically have a substantial boron content (about 15-30 atomic %) to be amorphous in as-deposited state. MgO layer in the CoFeB/MgO/CoFeB system crystallizes first during annealing at a temperature about 250° C. or above into stable bcc structure with preferred (001) orientation. Then the crystalline MgO layer acts as a template during crystallization of the CoFeB layers because of a good lattice match between the bcc MgO and bcc CoFeB. Annealing can also promote an interfaces sharpness in the CoFeB/MgO/CoFeB multilayer.
Crystallization of CoFeB is a thermally activated process. Therefore annealing at lower (higher) temperatures for longer (shorter) periods may provide similar results. The crystallization of CoFeB requires a reduction in the boron (B) content in the layers. The boron diffuses through the multilayer structure forming other borides with another layers of MTJ stack.
The diffusion of the boron into the MgO tunnel barrier layer may lead to both degradation of TMR and increase of the spin-polarized switching current. In order to reduce accumulation of the boron in the MgO layer during annealing, boron “getter” layer (or layers) may be inserted into the MTJ structure. The reduction of the boron content reduces the crystallization temperature of the CoFeB layers and promotes their transformation from as-deposited amorphous into bcc (001) crystalline structure starting from MgO interfaces.
There are two types of magnetic materials used in the MRAM: materials with in-plane or perpendicular magnetization orientation (or anisotropy). MR elements with a perpendicular orientation of the magnetization (or perpendicular material) have excellent thermal stability and scalability. Moreover theory predicts that perpendicular MR element can have substantially lower density of the switching spin-polarized current than similar MR element using in-plane magnetic materials.
Perpendicular MR elements of MRAM require TMR about 100% or higher and a density of spin-polarized switching current about 1·106 A/cm2 or lower. These parameters can be achieved in MR element wherein the tunnel barrier layer and adjacent magnetic free and pinned layers form a coherent bcc (001) texture with sharp and flat interfaces. This texture can be formed by annealing substantially amorphous in as-deposited state CoFeB/MgO/CoFeB multilayer at a temperature about 250° C. and/or above.
The pinned layer of the perpendicular MR element can have a multilayer structure for cancelling its fringing magnetic field produced in the vicinity of the free layer. The fringing filed can affect the thermal stability and switching current of the MR element. The pinned layer having a multilayer structure may comprise a layer of a ferrimagnetic material made of a rare earth-transition metal (RE-TM) alloy such as TbFeCo. Besides the pinned layer can comprise a synthetic antiferromagnetic (SAF) structure.
The pinned magnetic layers with the canceled fringing magnetic field can suffer from a poor thermal stability. For example, the RE-TM alloys may lose their perpendicular anisotropy at the annealing temperature above 200° C. The SAF structures frequently employ an ultrathin nonmagnetic spacer layer having a thickness less than 1 nm. The spacer layer is usually positioned between two magnetic layers forming the SAF structure to produce a substantial antiferromagntic exchange coupling between the magnetic layers. That is essential for stable perpendicular magnetization direction of the pinned layer made of CoFeB. Annealing of the SAF structure at the temperature about 250° C. and above may cause un uncontrollable diffusion through the ultrathin spacer layer and reduce the exchange coupling between the magnetic layers. That may lead to substantial reduction of TMR and increase of spin-polarized switching current.
Industry-wide efforts are underway to increase the TMR and to reduce the density of the spin-polarized switching current in the perpendicular MR elements for effective integration with CMOS technology. The present disclosure addresses to the above problems.